DEMAGNETIZING FIELD EFFECTS ON HIGH-RESOLUTION NMR-SPECTRA IN SOLUTIONS WITH PARAMAGNETIC IMPURITIES

The demagnetizing field is expressed as the sum of a diamagnetic term due to the molecular electronic magnetic moments of the solution which are induced by the external field and of a term arising from the electronic spins of the paramagnetic impurities. The effects of these two contributions of opposite signs on the NMR line are calculated as a function of the diamagnetic susceptibility of the solution, of the concentration of the paramagnetic molecules and of the sample geometry with respect to the external field. The theory is illustrated in D2O solutions by a spectroscopic study of the (CH3)4+ protons coupled with manganous Mn2+ high spin paramagnetic ions. Considerable shifts and linewidths of several p.p.m. are observed when the geometry of the vessel is modified and, except for spherical samples, the lineshape is asymmetrical. A good agreement with the theory is obtained.